Loop or folded dipole antennas include simple circular or square loops, whose impedance is readily calculated. An example of a prior art circular loop antenna 100 is shown in FIG. 1 comprising a loop portion 101, gap 102, and input/output connections 103.
Antennas of resonant size, where the size is in the vicinity of a half wavelength, have been typically fed at one feed point with series or shunt structures. In series fed structures, a signal is fed to one side of a gap formed in the conductor loop. Signal feeding to a series structure is considered to be a voltage generator. In a shunt fed structure, a signal is fed to conductor loop at two points without creating a gap. Signal feeding to a shunt structure is considered to be a current generator.
FIG. 3 shows a well known toroidal radiation pattern for a dipole antenna. It is readily appreciated that an electrical component of the electromagnetic wave forms a vertical radiation portion and the magnetic component forms a horizontal plane portion.
The design of non-traditional shaped antennas is not easy to analyze and are approximated by various design parameters. Variations away from traditional antenna structures have resulted in low efficiency antennas. In the past, non-traditional designs have been used in low frequency applications. As a result, necessarily poor performance of these antennas was adequate and was acceptable.
FIG. 2 shows cylindrical antenna 104 comprising a cylindrical band 105 with a gap 106 and feed points 107. The circular or tubular radiator of FIG. 1 is replaced by a larger surface area cylinder band 105. The radiation surface of band 105 is clearly increased over that of loop 101 of FIG. 1. For the antenna of FIG. 2, current distribution becomes concentrated opposite the input/output terminals 107, creating an undesirable cardioid pattern, i.e., providing negligible radiation to a substantial portion of an outward radiation plane leading to an undesirable asymmetric antenna radiation pattern. The prior art teaches that multiple feeds should not be made to an antenna where such feed points are less than a wavelength apart as coupling between/among the feed points is strong and in essence eliminates the benefits of multiple feeds, i.e., that the radiation from such an antenna would be the same as if it were made by one feed.